Treatment with lipid A or monophosphoryl lipid A abolishes the expression of suppressor T cell (Ts) function, without adversely influencing the expression of other T cell activity. This is due to the number and location of fatty acyl groups present, as well as to fatty acyl chain length. The inability of the lipid A of Pseudomonas aeruginosa to abolish Ts function, in conjunction with the capacity of inner-core region polysaccharides of its lipopolysaccharide to stimulate Ts activity suggest an important virulence mechanism.